![]() The stimuli are recognized by the region of the human brain which handles olfaction. This is the detection of stimuli by receptors in the nose. The perception of an odor effect is a two-step process. Most odors consist of organic compounds, although some simple compounds not containing carbon, such as hydrogen sulfide and ammonia, are also odorants. This does not correspond to the concentration or intensity of any single constituent. The olfactory system does not interpret a single compound, but instead the whole odorous mix. The patterns of neuron signals help to identify the smell. Different odorants are recognized by combinations of receptors. A single odorant is usually recognized by many receptors. Odor sensation usually depends on the concentration (number of molecules) available to the olfactory receptors. Olfactory information is further processed and forwarded to the central nervous system (CNS), which controls emotions and behavior as well as basic thought processes. ![]() The olfactory bulb acts as a relay station connecting the nose to the olfactory cortex in the brain. Interpretation of the smell begins there, relating the smell to past experiences and in relation to the substance(s) inhaled. When an electrical signal reaches a threshold, the neuron fires, which sends a signal traveling along the axon to the olfactory bulb, a part of the limbic system of the brain. Odorous molecules bind to receptor proteins extending from cilia and act as a chemical stimulus, initiating electric signals that travel along the olfactory nerve's axons to the brain. Each neuron has cilia in direct contact with the air. There are millions of olfactory receptor neurons that act as sensory signaling cells. The olfactory receptor (OR) cells are neurons present in the olfactory epithelium, which is a small patch of tissue at the back of the nasal cavity. The perception of odors, or sense of smell, is mediated by the olfactory nerve. Odor control covers at a sewage treatment plant: Under these covers, grit and gravel are settled out of the wastewater.
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